Phase breaking in ballistic quantum dots: Transition from two- to zero-dimensional behavior

We determine the phase-breaking time τ_φ of electrons in ballistic quantum dots, from the aperiodic fluctuations observed in their low-temperature magnetoconductance. Our analysis shows that at temperatures close to a degree Kelvin τ_φ scales roughly inversely with temperature, reminiscent of electron-electron scattering in two-dimensional disordered systems. At much lower temperatures, however, a saturation in τ_φ is observed, with the transition between the two regimes occurring once the thermal smearing becomes smaller than the expected level spacing in the dot. We therefore suggest that the saturation results from a transition from two- to zero-dimensional transport, as the discrete level structure of the dot becomes resolved.